140 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



Professor Hall at first recognized the distinctness of Petraster, since 

 he wrote as follows (1870): 



"The character of Petraster here described, and illustrated in the 

 figure (Sa, pi. 9), gives an intercalated partial range of disk-plates, 

 between the adambulacral and marginal plates, which will separate 

 these forms from any of the Palasasters in my collection." 



On a later page, after an examination of the genotype, he added 

 a "Note on the genus Petraster" in which he concludes that the 

 specimen was injured duruig life, as shown by the unequal distribu- 

 tion of the accessory interbrachial plates, and further that one ray 

 is "without intercalated plates on either side." He adds: "This 

 view is sustained by the fact that the other parts have the ordinary 

 structure of Palseaster, and in all other respects the specimen agrees 

 with the typical Palseaster matutina." The genotype was studied 

 at Ottawa by the present writer and found to be as described by 

 Billings. The accessory interbrachial plates, though somewhat 

 irregular in development in dijfferent areas in the type species of 

 Petraster, are normal in position and a similar but more complete 

 development also appears in Lindstromaster and Palasterina. 



Petraster is distinguished from Palasterina as follows: The most 

 important feature is that the inframargmal plates of the former 

 are prominent, closely adjoming and short but wide, while in Palas- 

 terina they are small, not conspicuous, and globular, or subquadrate. 

 The next important difference is on the abactinal disk, where in 

 Palasterina there is an irregular ring of prominent basal radial, 

 and supramarginal plates. These are absent in Petraster. The acces- 

 sory interbrachial ossicles in the latter genus never attain the distal 

 region of the rays as in Palasterina where these pieces are also more 

 numerous. Palasterina is also more abimdantly spinose than 

 Petraster. 



ScTiucTiertia has no inframarginals as marginal plates, which at 

 once distinguishes it from both Petraster and Palasterina. It is 

 true that in ScJiucJiertia inframaiginals are also present but they are 

 usually not well developed and remain adjoining the adambulacrals. 

 These two columns are therefore not separated from one another by 

 accessory interbrachial plates as in Petraster and Palasterina. 



The primary structure of Petraster is that of Iludsonaster. So many 

 of the generic characters are common to both that the former genus 

 appears almost certainly to have been derived through the latter. 

 It is true that the central abactinal area of the disk in Petraster is 

 devoid of the large basal plates, a marked character in Hudsonaster, 

 but the other generic characters are all present in the former. Petras- 

 ter adds more or less numerous columns of radial accessory ambital 

 and accessory interbrachial plates, none of which are present in 

 Hudsonaster. It is these accessory ossicles which differentiate these 



